1. Field of the Invention
The present invention relates to a semiconductor device which stores multivalued data. The present invention also relates to a semiconductor device including a circuit formed of a storage element and a thin film transistor (hereinafter referred to as TFT) and a manufacturing method thereof.
Note that in this specification, a semiconductor device refers to a device in general which functions by utilizing semiconductor characteristics. An electro-optic device, a semiconductor circuit, and an electronic apparatus are all semiconductor devices.
2. Description of the Related Art
In general, a storage device (also referred to as a memory device) includes a memory portion which stores data and a peripheral circuit (a driver, a decoder, a sense amplifier, or the like) for reading/writing data from/to the memory portion. In a conventional storage device, an area required to store one bit is larger than the size of one switching element (typically, a field effect transistor). Therefore, an area required to store one bit depends on processing technique for manufacturing a transistor, which prevents realization of a large-capacity storage device.
In recent years, as application software gets complex or the like, large capacity and high integration are highly demanded for a memory.
Patent Document 1 discloses a cell structure of a memory which is provided with an impedance phase change film formed of an organic material between electrodes. The memory has a structure where the film thickness of the organic material or contact areas with an electrode are varied in one memory cell, and a writing voltage is set at a plurality of impedance state transition points in hysteresis characteristics, so that information which can be stored in one memory cell can be made multi-valued.    [Patent Document 1]
Japanese Published Patent Application No. 2001-189431